1. Field of the Invention
The present invention relates to image reject circuits. More particularly, the present invention relates to image reject apparatus suited for frequency synthesis applications where the bandwidth ratios exceed 200 to 1 and the input data stream is a low frequency digital signal being mixed with a much higher radio frequency signal.
2. Description of the Prior Art
Image rejection circuits are known in the prior art, however, such circuits have a relatively narrow modulation frequency bandwidth which does not exceed a bandwidth ratio of 200 to 1 and are not suitable for synthesis applications. Further, the phase tracking degradation versus modulation frequency is generally unacceptable for frequency synthesis applications requiring linear phase-versus-modulation-frequency.
The simplest form of prior art reject apparatus employs a single mixer having a modulation signal input and an RF signal input to produce a pair of side band signals that occur on either side of the RF center frequency. By filtering the output, one of the side bands can be removed leaving the desired output signal which is usually coupled to a phase-locked loop as the input signal to the phase-locked loop.
Another form of image reject circuit employs two mixers at the input. The RF signal is applied as an input to both mixers. One of these mixers is provided with an RF input and the other mixer is supplied with a 90.degree. phase shifted RF input. The output of the two mixers is recombined in a power combiner employing phase cancellation techniques. The network employed to phase shift the modulation signal restricts the bandwidth and if a different bandwidth output is desired the reject circuit must be redesigned for specific bandwidth ratios and is incapable of covering a ratio in excess of 200 to 1.
A more sophisticated prior art approach to image rejection employs a quadrature hybrid circuit in the modulation signal path and a second quadrature hybrid circuit in the output path of the two mixers to produce a true sum and difference pair of signals or side bands. This image rejection circuit employs phase cancellation techniques as mentioned above. The phase shifter of the modulation signal is limited in bandwidth which limits the bandwidth ratio to a value less than 200 to 1 similar to the circuit described above. When an image reject circuit is desired for frequency synthesis applications requiring a large modulation frequency range in excess of 200 to 1, the prior art circuits described above are not operable with frequency synthesis applications because they constrain the modulation bandwidth. Furthermore, the output signal phase and amplitude from the prior art object circuits is degraded due to frequency dependent phase and amplitude variations of the 90.degree. phase shifters used in prior art circuits.
It would be extremely desirable to provide an image reject apparatus and circuit for frequency synthesis applications having a modulation bandwidth in excess of 200 to 1 and which will provide minimal degradation of the phase of the modulating signal.